I always took it to mean that like many philosophers, it is easier for Ishamael to be distracted by faulty logic because just because something makes sense on paper or when you think about it, doesn't mean it follows in the real world.

For example, you could go through many complex convolutions of thought and philosophy to conclude that 1 + 1 does not equal two, depending upon your manipulation of the numbers and the statements, so that the conclusion, while technically correct, may be wrong in the real world.

I think Ishamael/Moridin has always proven to be high with book or scholarly intelligence but exceptionally low with his understanding of people, which poisons how he thinks of the Pattern.

Basically, I think he's a very intelligent idiot who only made it as far as he did because of his strength in the One Power.